Gas exchange valves of an internal combustion engine can be operated in a variable manner, with different opening and closing times and with different valve opening lifts. A valve control system of this type is previously described in DE 42 30 877 A1. In this document, a cam support having two different cam contours is arranged so as to be rotationally engaged but axially displaceable on a camshaft. Depending on the axial position of the cam support, one cam contour is operatively connected to the lift valve via an intermediate member (transmission lever). During the base circle phase, the cam support is axially displaced by means of a thrust collar, counter to the action of a pull-back spring, to change the valve parameters.
A drawback in this instance is the large amount of space required for adjusting the cam support. These solutions can therefore only be used with relatively large cylinder spacings, in such a way that the relevant components can be accommodated. A further drawback is the high inertial forces during the adjustment process which are required for displacing the cam support or the adjustment members. It is only possible to switch to a corresponding cam contour in a cylinder-selective manner. Valve-selective switching is not possible.
DE 100 54 623 A1 describes a device for switching a cam support on a camshaft for actuating gas exchange valves, in which the cam support is guided in axial displacement on the camshaft. The gas exchange valve is operatively connected to different cam contours depending on the position of the cam support. The cam support is adjusted via an adjustment element in cooperation with a slide path. In this instance the adjustment element is a radially outwardly displaceable pin, which when extended cooperates with at least two slide paths formed in a guide part arranged through approximately 180° around the cam support.
A drawback of this solution, in addition to the additional space for the guide part, is that to switch to a different cam contour, the pin has to be extended from the camshaft and slid into an axially displaceable switch slide. After the switching process, the pin has to be retracted again. This construction has high parts and manufacturing requirements, and there is a risk of damage to the camshaft as a result of incorrect switching of the pin. A further drawback is that the necessary adjustment time of the pin restricts the rotational speed of the motor. In addition, the adjustment is dependent on the oil pressure provided in each case.
Further, a valve train of an internal combustion engine is described in DE 195 20 117 C2, in which an axially displaceable cam support having at least two different cam paths is arranged rotationally engaged on the camshaft. The cam support is adjusted by means of an adjustment member, which is guided inside the camshaft. A double-action hydraulic or pneumatic piston cylinder unit, arranged on an end face of the camshaft, displaces the undulating adjustment member inside the camshaft counter to the compression of a spring. The adjustment member is connected to an entrainment piece, which penetrates through a slot arranged axially in the camshaft and extends into a hole in the cam support.
The drawback of this solution is that by axially displacing the adjustment member, it is only possible to displace a plurality of cam supports arranged on the camshaft simultaneously. Different switching of individual cam supports on a camshaft is not possible. A further drawback is that in a switching position in which an external cam is engaged in the gas exchange valve, the spring element is constantly under tension. This results in high lateral frictional forces between the entrainment piece and the guide path arranged on the adjustment member. This leads to increased wear and possible related incorrect switchings. A further drawback is that the acting spring forces have to be adjusted precisely so as to prevent incorrect switchings, in particular when switching back to the central cam profile if there are three different cam profiles.
DE 10 2009 017 242, held by the Applicant, describes a valve train for actuating gas exchange valves of internal combustion engines. In the valve train, the cam support is displaced on the camshaft tube, for valve switching, by a switching shaft rotatably arranged inside the camshaft tube. The switching shaft is provided with a switching contour having an axial inclination. A switching ball, which is mounted in a hole of an axially displaceable switching sleeve which encircles the switching shaft, is guided in the switching contour. The switching sleeve is operatively connected to the cam support via a dog. When the switching shaft is rotated, the switching sleeve is axially displaced via the switching ball and the cam support is axially displaced via the dog.
The arrangement of a switching sleeve between the switching shaft and the camshaft tube leads to frictional forces which additionally have to be overcome. Moreover, the solution by way of the switching sleeve arrangement has high parts requirements.